Book - School of Science and Technology

522 Calculations for air-conditioning designAir mixture entering plantThis quantity is the air supply mass required to deal with the sensible heat gain and whichincludes both the ventilation air and the exhaust air from the hall which is recirculated.The mixture therefore has components having different properties of temperature andenthalpy, etc.From results above, by difference:mass of air recirculated ˆ (15:52 4:82) ˆ 10:7 kg/sThus, temperature of air mixture, dry bulb[(10:7 21) ‡ (4:82 28)]/15:52 ˆ 23:2 Cand enthalpy of air mixture[(10:7 41:08) ‡ (4:82 51:95)]/15:52 ˆ 44:46 kJ/kgAir mixture leaving plantIt is now necessary to take account of latent heat gain in the room inasmuch as this willaffect the ability of the air mixture leaving the plant to absorb unwanted moisture in theroom.Latent heat gain in room as previously:calculated ˆ (3:8 ‡ 15)ˆ 18:8kWFrom p: 2, latent heat of vaporisation of water at 21 C ˆ 2450 kJ/kgThus, moisture increment in room:18:8/(15:52 2450) ˆ 0:00049 kg/kgFrom Figure 18:1, moisture content of air at room condition ˆ 0:0079 kg/kgThus, moisture of air leaving plant:(0:0079 0:00049) ˆ 0:0074 kg/kgThe energy used to drive the supply fan will be converted into heat and a proportion ofthis will be transferred to the air stream. Also, the walls of the supply duct betweenthe plant and the room inlet, although thermally insulated, will allow some heat ingress.As a result, the dry bulb temperature of the air leaving the plant must be lower thanthat required at the room inlet. Such heat gains may be assessed in detail but, for thisexample, may be taken as 10% of the sensible gain, thus accounting for a 1.0 K temperaturerise.Hence, at the inlet to the supply fan, from Figure 18.5:dry bulb temperature ˆ (15 1) ˆ 14 Cand, from Figure 18:1, air at 14 C dry bulb moisture content ˆ 0:0074 kg/kg, thenwet bulb temperatureˆ 11:5 Cand enthalpyˆ 32:80 kJ/kgCooling capacityFrom the various values now available, the amount of cooling capacity required to meetthe required conditions for peak gains in summer may be calculated.